Something prevents me from reaching no space left on iPod
I don't know if this was available on iOS 9.5 but on iOS 11 there is an option to conserve space on ios devices. What this does is remove apps that you haven't used to make rooom for new apps or files. I suppose it only removes Apple Store apps since you can easily restore those. this sure sounds like what your iPod may be doing. On my iphone this is turned on by going to Settings/General/iPhone Storge. There is an option there to turn it on. To turn it off go to 'iTunes and App Store' and scroll down to Offload Unused Apps and turn it off there.