If you have also upgraded the FW maybe the default config has changed. Read this and check your config:
If not it could be a bug and maybe raise a case to the support but first you can check the globalprotect pangps, pangpa logs:
I also see that globalprotect 5.2.8 is really new and there is nothing about known or addressedissues that look like yours so if needed the next step will be to contact the TAC for possible new bug:
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