Steps to reproduce:

There is not possible to do simple result of subtraction in calculated item - for example:
I have simple file somefile.txt that contains only static value "20", then I do calculation in calculated item:

last(/Zabbix server/logrt[/somefile.txt])-10-2

result: 10, but should be: 20-10-2=8

another test:

last(/Zabbix server/logrt[/somefile.txt])-10-2-1-1-1

result: 10, but should be: 5

but this works properly:

last(/Zabbix server/logrt[/somefile.txt])-10+2

result: 12, it's ok: 20-10+2=12

another test:

last(/Zabbix server/logrt[/somefile.txt])-10+2+1+1+1

result: 15, it's ok: 20-10+2+1+1+1=15

So, problem is only in substraction. In Zabbix 5.0.x all works fine, but not n Zabbix 6.x.x (my version is 6.0.3)