Time machine full : oldest files not deleted

Time Machine removes the oldest qualifying backups but does not ransack the backups attempting to remove as much storage as needed to achieve the current TM backup. Although the size of your internal drive plays a role in the initial capacity planning for TM drive size (2.5 - 3x), it is what has changed on your Mac since the last successful TM backup that determines whether the TM backup can succeed with the remaining storage on it.

It should delete but if not you can always start a new drive (or erase the present and start again). I have done this many times.