In datasheets, you usually see 2 sets of ratings:

1. Operational
These specify the conditions (eg, supply voltage) under which the device is guaranteed to work within its specifications.
This does not mean that the device will not work outside those limits; just that there is no guarantee that it'll work at all, or correctly, or reliably, or for its normal life expectancy, or be in spec...

2. Absolute Maxima
These specify stresses which the device is guaranteed to survive.
Again, this does not mean that the device will not survive beyond these limits; just that there is no guarantee that it will work at all, or correctly, or reliably, or be in spec afterwards.

So, it looks like you've just been very lucky!

As with anything operated outside its specified limits, there is absolutely no guarantee whatsoever that your device will continue to work - it could fail at any time, or start exhibiting any sort of weird behaviour at any time!

If you have stressed the device beyond its Absolute Maximum limits, this could still happen even after you fix the 5V regulator - you may have caused damage which may not exhibit symptoms immediately.

Its behaviour is now effectively UNDEFINED!!

Therefore, if this is an important system, you need to fix that regulator and replace the 89c52 immediately!