This isn't all together uncommon. Maybe now try just swimming the 50m with a focus solely on ease without regard to spl. That's not to say to swim 'slowly' but do it with deep concentration in particular with regards to your breathing; rhythm.
Now try it again with the same effort but this time count your strokes. Don't try to hit any particular spl just see how many you take with this concentration. Once more.
Don't be alarmed if your count is back up. How was your breathing? Breathless? Over all sense of balance and alignment?
Now perhaps try another length trying to reduce your count by one. Do this with the same effort and attention to balance and 'slipperyness'. How's your breathing? Over all sense of balance and alignment?
Maybe try continuing this process always mindful to keep the same effort bringing the count down only by your 'intent' not by straining.
The minute you feel breathless take it back up (spl) and refocus on your ((effort)); overall sense of balance alignment and slipperyness.
As mentioned the 'strain' can keep you away from air more than you'd like without realizing it.
Perhaps mark your "strained" spl and your "effortless" spl and work to close that gap not by trying but by swimming more efficiently thus effectively.
50m sounds like a good repeat distance for this practice adding timed intervals or breaths (while resting). Soon you'll decrease the intervals an subsequently the distance will increase ... with ease : )