This right is here is the precise issue WotC has been dancing around all Edition: tons of people play in the FR, buuuut...many use the 1E timeline, many use the 2E timeline, many use the 4E timeline, many use the 5E timeline and many more are in a super divergent alternate timeline.
The 5E adventure and setting books have been very carefully trying to be useful to all of these groups and Homebrewers looking for material to repurpose.
They've done a great job with that balancing act, but it makes it difficult to commit to a firm canon again...