Problems page, tags Filter:
"Contains" operator is NOT case sensitive. That means if condition is:
tag Contains aaa
the result is:
Problems with tag=aaa and tag=AAA.
"Equals" operator IS case sensitive, so if condition is:
tag Equals BBB
the result is
Problems with tag=BBB
and NOT tag=bbb.

Inventory -> Hosts
Both "Contains" and "Equals" operators are NOT case sensitive,
so filter OS contains bbb
will result with OS=bbb _and _OS=BBB
the same as filter OS equals bbb.

Configuration -> Actions -> Create action (All Event sources)
Both operators "Contains" and "Equals" as much as "Does not contain" and "Does not equal" ARE case sensitive.
So condition Hostname contains "host" will not match with Host-1.

Configuration -> Event correlation -> Create correlation
Both operators "Contains" and "Equals" as much as "Does not contain" and "Does not equal" ARE case sensitive.

Configuration - > Maintenance -> Create Maintenance -> Hosts and Groups tab. Tags Filter
Both operators "Contains" and "Equals" ARE case sensitive , which differs also from Problem page tag Filter, so it definitely is a bug.

Expected result:

All operators over all Zabbix UI should work consistent.
"Contains" (also "Does not contain") operator everywhere should not be case sensitive, and
"Equals" (also "Does not equal") operator should be case sensitive.