Hi,
I think there is a default limit when fetching post tags in the wordpress get_terms function, it is around 400 if I recall correctly, so that might be the issue here.
Interestingly it never came up until now, I am more than convinced that this was like that for years for sure. I have quickly made an issue and already merged a solution to increase that to 10 000 in the next release.
If you want, you can add temporary FTP access and I will make the change for you, or suggest the modificaiton. It affects only a single line of code.