How diff() works in alerts

Hi, i’m trying to manage alerts when some query change the values.
my query A reads one table on mysql db on aws. that table changes values only once a week. i want an alert when that value changes. so my solution was setting a alert like this:

diff ( query(A,5m,now) ) above 0

i did that thinking about this example:

value on monday at 9:00 —> 100
value on monday at 9:05 ----> 120

when the values changes the diff is above 0 so the alert is true and it fires

the problem that i have is this

value at monday 9:10 ----> 120
value at monday 9:15 ----> 120

the value doesnt changes but alert keeps firing.

i hope that all the info is clear. i would need help with this.