You should use the worst progression when defining maneuver progression, not the best progression. You multiclass to be more versatile, not to multiply power. That's the whole point of multiclassing IMO. Single class should always be more competent in their niche than multiclassed characters.
the biggest problem with this idea is that it leads to odd edge cases where you might actually just straight up lose the ability to use maneuvers you could use before because you took a level in a class that, at that point, would only have access to maneuvers of a lower degree then you already have (the easiest example that comes to mind is something like fighter 17/rogue 1, where suddenly you can't use 5th degree maneuvers anymore because rogues can only get up to 4th degree).
The ideal would be a table like spellcasters have that defines multiclass progression, but we're stuck with RAW, and I don’t think new material should deviate from it too much. But yeah, a Herald stands to gain way too much from a single level of Fighter.
i'm not entirely sure why they DIDN'T do this, because RAW (as we're finding out) it's a mess. maneuver progression between classes is basically already unified anyway - from the adventurer's guide, heralds and rogues (and savants, but they get explicit declarations from an optional feature and not a maneuver progression table, so i'm not entirely sure if they count) are 2/3 martials (they end up at ~13th level fighter maneuver wise - they're locked at each degree of maneuver for 1.5 times that of a fighter), fighters and marshals are full martials (marshals don't have maneuvers at 1st level, like the herald and rogue, but also like the herald and rogue this doesn't actually impact its maneuver progression), and every other martial is a full martial minus 1 level. the math is a little gross, but it's workable. i can even write something up right now:
"Your highest degree of maneuver is determined by adding all the class levels you have from classes with the Combat Maneuvers feature, but some classes do not count as highly as others when determining your highest degree of maneuver available.
Your class levels in the fighter and marshal classes count fully towards your highest degree of maneuver available.
Your class levels in the adept, berserker, and ranger classes also count fully towards your highest degree of maneuver, except for the first level in each class - your effective level in such classes for the purpose of determining the highest degree of maneuver you can take is thus one less then the level you have in that class (minimum 0).
Two thirds of your levels in the herald and rogue classes count towards determining your highest degree of maneuver."
this is basically just the spellcasting levels section but for maneuvers. the multiclassing table would just be the fighter's.
and for future proofing:
"Future classes or subclasses may have different rates of progression for determining the highest degree of maneuver you can take. To determine this progression, look at the stretch of levels for which the class' maximum degree of maneuver is second, and divide the same stretch of levels from the fighter by this number. For example, the herald can access only up to second degree maneuvers for 6 levels, while the fighter has this restriction for only 4. The herald's progression is thus 4/6, or 2/3. Additionally, look at the overall progression to see if the progression of maneuvers is delayed at all, and if so, subtract the number of levels by which progression is delayed from class levels for that class when calculating its total level BEFORE applying the overall rate of progression."
so the fighter 3/herald 10 example from the book using these rules would be effectively a level 9 fighter (fighter 3 + herald (10*(2/3)) or fighter 3 + herald 6) with access to 3rd degree maneuvers, not level 13 with access to 4th degree maneuvers (compared to a herald 13 being equivalent to a level 8 fighter with 3rd degree maneuvers). this makes a lot more sense to me, and it isn't really that much more complicated then figuring out caster levels.
