see https://www.zabbix.com/documentation/3.2/manual/appendix/triggers/functions for an example (all versions affected)

for the change() function, the comments say :

For example:
(previous value/last value=change)
1/5=+4
3/1=-2
0/-2.5=-2.5

slash - '/' - is most often associated with division. thus 3/1 would be 3, not -2.
while here it is used to separate prev/last values, it is rather confusing. it might be better to change the comment like so :

For example:
(last value - previous value = change)
5 - 1 = +4
1 - 3 = -2
-2.5 - 0 = -2.5