PowerApps will wipe out custom formulas for virtually every control in the app that uses them for X, Y, Width and Height whenever one changes the Screen size and orientation settings within AppSettings under the File menu.
Formulas such as:
X = ( Parent.Width - Label1.Width) / 2
Width = Parent.Width / 2
(or just about anything), INSTEAD will get replaced with numbers (like 383.0 or 615.932), wiping out the formulas, and generally messing up the entire application.
This is a MAJOR pain, and dramatically wastes time fixing when one really does need to change the Screen size and orientation values in App Settings (we're talking HOURS in my app, been there done that).
WHY does PowerApps automatically change formulas to constant values WITHOUT NOTIFYING THE USER BEFORE HAND and giving an option to NOT make those changes?? This is irresponsible.
Note that this is FAR from the first time and not close to the only case where this happens. It happens ALL over PowerApps - I REGULARLY have cases where my app suddenly have 50-100 errors when I open it up as PowerApps has "conveniently" changed my code to something that just is an error...
For instance the latest one (other than the above) is where I have a collection with multiple members, and it will rename accesses to one of the members with some mangled ID EVERYWHERE that member is accessed, and throw and error each place.
ie: say a collection cMyCollection has an id property, and in my code I access this as cMyCollection.id. This could be working fine for weeks without PowerApps changing it, but then suddenly one day I load my app and I have 50+ errors (one for every time cMyCollection.id was referenced) where all those were instead changed to something like cMyCollection.'8912eabd83b13a2id' and I have to MANUALLY change all of these (note: the "id" is still there at the tail), however other members like name (cMyCollection.name) aren't affected. I change these, save, publish and all is well for a while - sometimes hours, days, or weeks, and then this comes back. I have just gotten into the habit of "biting the bullet" and making these changes manually (and I'm getting quite fast at it).
Note that "id" isn't the only member name (in case this is something "special") that this happens to.
Also - while lately I have seen this in collections, in the past I have seen similar things in data sources, though not for a while (other than the occassional change from fixed field name (ie: crac4_name) to display name 'Name' without warning and without changing settings - but at least the app still "works").
Please fix these issues. One of the MANY frustrations with PowerApps that makes me literally hate it as an environment.
I found this problem as well, the hard way.
I should note that also notable is the property Size. (as you said, any formula in a dimension-related field)
PowerApps needs to give better support for the advanced use case of dynamic/responsive layouts.