To be frank, I don’t think that was wise. You’ve functionally given a 3rd level character a very rare +3 weapon, even if only for a brief time. If the Narrator is inexperienced and permits that kind of exploit, you have massively imbalanced the game. Assuming a fight against a CR 1 creature with AC 13 and 30 hit points and a berserker with a +5 to hit at level 3 using a greataxe for 1d12+3 damage (which should be a fair 1 on 1 fight), it should take the berserker about 4.8 turns to take the enemy down. With an extra +3 to attack and damage, that changes to 3 turns on average for the berserker to win. You’ve functionally given the berserker 1.8 extra turns, which means that any other enemies in the fight are also going to go down faster because now the berserker can team up with their party.
More importantly, if the Narrator does rule against it, and the player was hoping to use it, the relationship between the Narrator and player has soured because the Narrator “had to take away the player’s fun” rather than the designer anticipating that and making it a non-issue at the table.
If I may ask, who do you have to review your designs with? Aside from posts to announce new products, you don’t seem to be very active on the forums unless you also go by another name for non-publishing content, and I also don’t see you on Discord that much as far as I know. I ask because if you’re on your own, I want to invite you to take advantage of the extremely approachable and kind community of professional designers (and long-time enthusiasts with sharp noses for good design) around here who would be thrilled to go over drafts with you and help you put out the best products you can. More than that, they’re all just people who are worth having as friends.
Also, if you’re not on it already, you may want to ask Savannah to put you on the list for GPG Pitch calls so you can submit some designs for official inclusion. Having one or two official contributions to the game will strengthen your brand as a 3rd Party designer and also help you make more connections in the design community. You’re obviously very creative, and I’ve been really impressed with just how much content you’ve been able to put out in so short a time! I think that you have the potential to really make astounding contributions to this game, and I just want to make sure you’re in the best position to do so.