I hadn't really heard of the bug before, so I don't know how long it has been going on for.
I have edited magizoology levels on my main and alternate account, and also on a few friends accounts. The value is
usually between 1-100000 if you have not maxed it out yet (the value does roll past 100000 if you are max, but that doesn't affect your current cases). The value itself is cumulative, meaning that it is the sum of all previous levels. I doubt that this number will fix itself, but there are 2 ways that you can fix this if you are using game guardian.
The first is to find the value and edit it directly, which would be my prefered method. If you are having trouble locating the exact correct value (could be possible as the -30000ish value could be your current levels xp and not your actual exact cumulative value), you can use range search. It's the same as an exact search, but you could type it as, for example, -50000~-1. This will find all values between -50000 and -1. You can the do a fuzzy search for changed values every time you gain more xp to narrow it down to one value. I can usually narrow down the correct value within 3 searches, so have at least 3 creatures ready to feed/bond.
The second method is a bit slower, but does remove the need to find the actual value. You can try to find the value for the bond timer for a creature (the value is the number in seconds). You can use the "Creatures" tab from
this document to find the bond timer for a creature (from lines 100 down). For example, a 12 hour bond timer is equivalent to 43200. You would do an exact search for 43200, select all the values and then change that to 1. This would temporarily make the bond timer 1 second for creatures whose previous bond timer was 12 hours. You could then just continuously bond to raise you magizoology level back up to the proper number.