Half black frame before lower third graphic in both playback and export

2019 27" iMac

2TB Fusion Drive

3.7 GHz Intel Core i5 

32 GB 2667 MHz DDR4

Radeon Pro 580X 8 GB

Running OS 13.1

Premiere Pro 2022

I have created my own lower third graphics in a sub-sequence which is full-frame. Then I put the sub-sequence onto the main sequence, crop the borders down and re-size and place it where it needs to be. In playback, it will usually show a half-frame of black in the frame immediately preceding the graphic dissolving in, but of course when I jog through that section, the half-frame of black is not there. And if I play it back again, usually there is no black frame. But after export, there it is, a half-frame of black preceding the dissolve-in of the lower third. I've tried creating the graphic different ways, and still get that half-frame of black. My next course of action will be to export each graphic with the alpha channel, and try inserting the graphics that way. But I shouldn't have to!  This is infuriating! Please help!