Had a look at the schematic for this since the issue would have to be something related to both cameras. It is possible that there was electrical damage done during the repair, but that sort of damage does not occur very often to cameras. I am more likely to think that there may have been some accidental damage done when disconnecting the screen.
There is at least 1 line on the board which is shared by both front and rear cameras and its possible that components related to this line were nudged when you disconnected the screen since the components are very close to home button/force touch connector. Which is vertical connector coming from the screen. I might see if anything in this area (especially around the upper right corner, near the screw hole, looks like it may have been disturbed. You can remove the foam stickers around the connector for a better look. And you may need some level of magnification to be able to tell anything.
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