Coalescing triggers of the same name in monitoring overview

[tgsbn]

I have recently joined a sysadmin team using Zabbix 4.0.23 for monitoring.
There are many items configured with several triggers corresponding to different thresholds for different severities, all with the same name.
For example, the item "backup" measuring how many days ago a backup medium has last been changed, would have three triggers, all named "Backup_History":

• one with severity High and expression "backup.last(0)>18"
• one with severity Warning and expression "backup.last(0)>14 and backup.last(0)<19"
• one with severity Information and expression "backup.last(0)>10 and backup.last(0)<15"

The intention, according to my colleagues, was that the Zabbix monitoring overview would show a single line with that name and colour every column according to the highest severity trigger that matched for the corresponding host, or green if none.
They also tell me it actually used to work that way.
However right now it doesn't, and nobody knows why.
The monitoring overview shows three lines named "Backup_History".
Worse, those three lines do not even correspond to the three triggers.
For some hosts, the High severity trigger is in the first line, for others in the second or third line, same for the other severities, following no discernible rule.

What could be the reason for this behaviour?
How can the wanted behaviour be restored?

And an auxiliary question concerning the example triggers I quoted:
Is it really necessary to make them mutually exclusive in that way?
Wouldn't a higher severity override a lower one automatically?

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Tilman