Sometime, rewrite rules interfere with custom error pages.
For most setup it should work as expected. but, sometimes, need some minor tweaks in LiteSpeed to make it work like Apache.
Please turn on debug logging, rewrite logging for the vhost has this problem, then produce the problem, send us the error log for analysis.
And, we need to get enough information to reproduce this problem in our lab in order to have it fixed, usually involving copy over configurations and related files.