This kind of thing happens to apps regardless of whether they are closed source or FOSS.
If management of a closed source app in an app store changes in a way that you don't like, you have zero recourse.
If it happens to a FOSS app, then you could potentially use the source to install the app yourself or pay another supplier to do it for you.
Although given the reality of the low cost of mobile apps and small, niche user bases, your purchase probably amounts to something more like contributing to a tip jar anyway.
If management of a closed source app in an app store changes in a way that you don't like, you have zero recourse.
If it happens to a FOSS app, then you could potentially use the source to install the app yourself or pay another supplier to do it for you.
Although given the reality of the low cost of mobile apps and small, niche user bases, your purchase probably amounts to something more like contributing to a tip jar anyway.