Putting shower head in the sink caused flood
If the water is not currently leaking that means that there is not an issue with a leaky pipe, but that water was flowing down the path of least resistance; i.e. the shower to the floor?

The circumstances that you have related seem to point to the shower head being placed on the floor as the cause of your damage. To rule out other causes it would be best to duplicate all possible scenarios (e.g. fill the tub and check for leaks; use the shower normally and check for leaks; lay the shower head on the floor and check for leaks, etc.)

Does the fact that the floor leaked mean that something is not caulked correctly or is it not possible for floors in bathrooms to handle a lot of water?

NO, unless purposely built as a &quot;wet room&quot; with a floor that slopes to a drain, bathroom floors are not waterproof and significant flooding will damage underlying structures.

I was able to put a knife pretty deep into the wood, can a single
water event cause the wood to get that soft or would it require
multiple events?

A single event might cause such damage but only if it was a significant event and there were conditions that prevented drying, and it would take a long time. That type of damage seems to have been caused by repeated events over time that kept the area moist.