What settings did you use for Slack, and did you, by chance, have `downclock_on_battery=` enabled?
https://github.com/kernc/xsuspender/blob/92ea5c6dfcb0f79eb10...
https://github.com/kernc/xsuspender/blob/92ea5c6dfcb0f79eb10...
[Slack]
match_wm_class_contains = slack
send_signals = false
IIRC, I still had issues with this setting