I discovered some time ago that the Forum does not seem to be indexing past March 18-19, as you have noted.
If you check my username and a few others as a comparative test, you will find that this is the case.
Perhaps the admins will fix it eventually, but there are probably lots of things to do to keep the Forums running smoothly.
In any case, this indexing issue is affecting all threads and users (as far as I can tell) since March 19.
Have a nice weekend!