Hello there,
Noticing that the Modern Form control has a couple of strange things with it that I wanted to bring up.
- When you first add a form, if you select the Data Source from the dropdown in the Properties pane on the right hand side it will automatically pull in all fields from your data source. (As I typed this, I was waiting for PowerApps to populate the form so I can get a screenshot of what happens for number 2, had to close and reload the app)
If instead you open the Data Source property at the formula bar and type it in, it will connect to the data source without populating all the fields. You can then pick and choose which ones to pull in. This is definitely the preferred result since sometimes we have a number of fields that do not need to get populated each and every time.
Is this by design? If so definitely leaning to option B, no fields populate on data source selection. Option A can end up never loading and being stuck on the spinner till you reload. - Adding a lot of fields at once seems to cause the form to 'black-out'. It seems the data card properties don't set quite right and just create black boxes in every data card and the control X and Y properties go haywire and just auto align to the top left corner.
Simple fix for this is to save, leave the app, then reload. Form seems to come around and go back to what you'd expect.

- Data cards of dropdown fields with multi-select enabled don't populate correctly. Similar to the 'black-out' in item two, they align top-left. Data cards with single-select dropdowns seem to be unaffected. Not a big issue as you can just drag or set the properties but figured worth mentioning. Saving and reloading the app seems to fix this as well.

My authoring version is 3.24012.8
To replicate any of this, simply add a Modern Form control and then attempt to connect to a data source using either approach.
An idea for a possible improvement would be if we could set the data card positions in a numerical fashion, like setting a position value that tells the data card what slot to sit in.
1 | 2
3 | 4
etc.
You can sort of do this now by changing the X and Y properties of the data cards but if you make one visible = false, the form holds a blank space instead of letting the rest of the cards move up. This only happens with more than 1 column. Single column doesn't do this.
Two Columns:

Single Column (the selected card is a text field, the card showing is date/time) fields move up:

Thanks!