Losing Variable

Hi.

I use a G30 to set a Z position for my tool change position.
I did a tool change a few days ago and the tool cycle did not lift the Z out of the way before turning the tool carousel causing a crash (this is what the G30 was doing in the past).
After investigation it turns out that the variable table (linuxcnc.var) had dropped the 5183 offset (G30 Z offset).
I reinserted the offset and carried on.
Today I rechecked the .var files just for interest and the 5183 variable had again been rewritten as 0.0000!!
Lucky I checked as it prevented any crashes, but I am at a loss as to why this variable being sometimes overwritten?
It has persistence as it usually stays in the table but it is worrying that it could simply disappear at random.
I noticed that the linuxcnc.var.bak table had the 5183 parameter as 0.0000 as well although I'm not sure of the relationship between these two files.

Thanks for any help on this issue.
Regards,
Marty.

It is hard to tell whether other variables are being zeroed out as all except one are zero anyway. The 5183 is literally the only one (except one other which is a1) which has a non binary value.
I like your option of checking the value of #5183 before a tool change. I also think I should re-write the value of 5183 to the variable table at the start of each tool change macro routine as well (#5183=96.52), as this variable is not dynamic in any way, and thus force the variable to have the correct value each time despite any unknown interim re-writing.
Another option might be to use G53 Z96.52 with the tool change move in lieu of the G30 as it is non modal and absolute for that one move.
Do you have any thoughts as to any preferred option on the above?
Thanks,
Marty.