2CD2132F-IS has a gap in footage on SD card

[cam235]

I recently got a 3MP dome camera, DS-2CD2132F-IS running v5.3.0 build 151016 and I put a Sandisk 64GB SDXC card in it. I don't have any external DVR, I'm just doing motion recording plus still frames every half hour to the internal uSD. I've checked the playback for several days and everything looks good. Today I noticed one motion video had a gap of 1-2 seconds just a moment after it started (the car blinked from one side of the frame to the other). This was also about the same time as the available disk space for videos went to 0 GB. The recorded videos before and after that look fine.

I had assumed this camera works like a dashcam where it continually deletes the oldest files to make room for new. However, if the timing of "delete the old" is wrong, does that cause a recording glitch? I've seen another thread saying H.264+ causes trouble, but as far as I know this model doesn't have that feature (?) At least the Configuration->Advanced Configuration->Video/Audio->Video->Video Encoding selector has only one option, "H.264" and for Main Stream I have "Video&Audio", 20 fps @ 2048x1536, 8192 Kbps, variable bitrate, highest video quality, Main Profile, I-Frame Interval 50, SVC OFF.

Before installing the microSD card I took the time to do a complete write & readback test using the h2testw program from www.ctmagazin.de, and it passed. The card is this one:
https://www.amazon.com/gp/product/B010Q588D4

Thanks for any insights!