Maybe this is not bug, but limitation of 4.0 against 3.4 and older.
As Monitoring > Triggers does not exists anymore in ZABBIX 4.0 (and I understand importance of this removal), its sub-feature of displaying all comments/acknowledgements for all problems generated by that trigger is now unavailable too.
Users of ZABBIX (customers) often use trigger in position of "problem type" and in many cases it makes sense to see past comments for (repeatedly occured) problem (resp. problem type). This information is often used as evidence of how was such problem handled by support staff in the past. For some, this is showstopper to use frontend 4.0 for handling problems by established practise.
Related to https://support.zabbix.com/browse/ZBXNEXT-4413.