This is quite tricky to answer in isolation, it really all depends how your overall routine looks and what changes you are making at the time and if it's a regular thing to wake at 50 mins.
For instance mine could wake up at a time after he had transitioned to his next sleep cycle at 40 mins but before his full nap was complete, he'd be upset, crying, and I could resettle him back to sleep. He was UT for his nap. Many times we say an UT baby wakes at the end of one sleep cycle and tends to wake relatively happily but it's not always the case. This is why it helps to look at the whole routine and what LOs habits are. A 50-55 min nap *could* be due to this. But it could also be down to something else, noise, too hot, too cold, an external sound waking him, pain, habit...
It could also be his natural sleep cycle length. Tracy said sleep cycles were 40 mins long (my DS had exactly 40 mins), often on the forums we see 45 mins (possibly more so than 40 mins) but sometimes we find a LO with a 30 min sleep cycle which can really cause a confusion and I've seen a few of those over the years. It's logical that if there are some individuals with a 30 min cycle then there could also be LO with a 50 min cycle, yk?
Sorry, I know it's not the straight forward answer you might have hoped for but I hope this helps to explain why it is often difficult to look at times in isolation of the bigger picture.
xx