Well, since this apparently happened all of sudden, it is very possibly due to a leak somewhere, or a failure in a recirc system if you have one..
NOW, you should know that by "rule of thumb" a 50 gallon heater will deliver approx. 35 gallons of water at suitable shower temperature. A 'medium long' shower will use up most of that, leaving little for that tub! And this situation is based on some assumptions: heater set at ~125 degrees, ground water ~55 degrees, so it will vary in the winter.
Certainly the dip tube would cause that. This is not a common problem, but is fairly quick to check.