Question about Sandberg Outdoor Solar Powerbank

I use mine quite often and always charge it in full sun. Now it has a bulge on the back since last time. My conclusion would be exploded battery. But then I noticed that it still charges my smartphone and also charges itself in the sun. Is it possible that "only one battery pack" exploded and another one is still working? (I have no idea how it's constructed inside :/) Or do you think the battery is still intact and the bulge is from something else? Would you send the powerbank in? Has anyone else had the same experience?

I used the powerbank for 8 days on an outdoor trip.
It worked great, outside temperature was about 25 degrees at noon.
But it does get quite warm.

I have not used the device yet

It will be a bigger load.
I have not noticed anything negative in this respect during a few weeks of use. Because of the solar cells, it must be built in such a way that it works.