Trunk Contact unreachable and reachable after restart

Dear all,

I am here hoping to find some answers. I have two trunks configured with pjsip and both have same settings. Both are showing green contacts in report and everything is working smoothly without any problem (Inbound and Outbound telephony works as designed). When I go back the other day and look at the trunk status, it tells me unreachable and red with Message: No contacts.
NOW: If I simply do a systemctl restart asterisk, everthing works again immediately without any problems.

As this is somehow unreliable and dangerous situation, does anybody have an Idea where it could come from or which settings I could check?

Regards