I thought I'd share this since it had me scratching my head for a while.
I have a simple project that interprets a signal received on a radio receiver module, displays the results on an OLED display then uploads them via GSM.
Everything works fine if I use the USB input to power the system but powering via Vin results in unpredictable and erratic behaviour of the receiver and OLED. Attaching or omitting the battery makes no difference.
It turns out that there is a design flaw in the MKR1400 which, unlike other Arduinos, does not have a voltage regulator on the VIN circuit. According to the specs VIN must be 5V.
On the MKR1400 (and probably other MKR boards as well) VIN connects via a Schottky diode (Q2B) which has a forward voltage drop of about 1V which means that the 5V bus becomes more like 4V which was the cause of my problems.
The solution for me was simply to bridge the 5V and VIN pins which are adjacent anyway.
A quick glance at the schematics doesn't suggest any issue with this, providing I don't plug in the USB at the same time but I would appreciate a peer review of the solution and possibly a comment from teh designer.