Did a full search on the 388.1 release thread and found only one other person mentioning the same problem you are having.
https://www.snbforums.com/threads/a...all-supported-wifi-6-models.82084/post-808955
WiFi driver and related issues are outside Merlin's control - so don't expect any help from him.
GPL version 388_
21224 on which 388.1 is based for your router - was never publicly released by Asus for the
AXE16000 ... so no way for us lesser mortals to determine what it may have contained by way of drivers etc. - or even what changes/fixes Asus may have included in it.
Have you tried Asus stock code 388.
21617 [more recent than what was included in 388.1] to see whether it works fine for your Guest WiFi?
That version contained a number of fixes which may or may not be in Merlinware 388.1
There is an even newer Asus stock firmware - 388.
22237 which included a few other fixes - but may be worth a try.
If Asus stock versions above work for your Guest Network issue - then all you can do is hope that the next released version of Merlinware 388.
3 will be based on a working foundation for your model when it comes to drivers and other closed source issues.