Sorry for the late response. In general smart phones like about 10% available memory. Phone has 16gb capacity, at least 1.5 go free. When it's less than that I find exactly what's happening to you occurs; phone gets poltergeists, stuff vanishes Etc. I'd start by backing it up and then delete stuff.
Source; work in advanced iPhone tech support.
Fancy Schmancy Custom Road bike ~ Mondonico Futura Legero
Found on side of the road bike ~ Motobecane Mixte
Gravel bike ~ Salsa Vaya
Favorite bike ~ Soma Buena Vista mixte
Folder ~ Brompton
N+1 ~ My seat on the Rover recumbent tandem
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