After Effects isn't Rendering Certain Frames when I do Time Remapping

I just encountered this exact same glitch with a mouth pack for a character I made using AE 2021. It renders is fine in the preview but skips it completely when rendering the video out with media encoder. 
I couldn't find a fix anywhere. 
Tried clearing memory and cashe multiple times, even re-drew the desired frame (using a traditional type mouth pack with time remapping and toggle hold frames).

Finally went ahead and used the solution you gave. Moved that frame to the end of the timeline in the pre-comp and went in and manually changed every frame with the "dead frame" to the new one.

thisnis the first character I've made in AE and followed a popular tutorial on YouTube for the mouth. 
I can't imagine I'm the only one having this trouble in 2021.

Anyway, thanks for the work around. Seems to work.

You can share any file stored in your creative cloud folder. You can use Dropbox or any other file-sharing site. Google Drive... Any location will do.

If you would like to upload your project I'll take a look. Seems weird to me. I have not seen this kind of bug.

Here's an example of the setup I have

1.) I have a pre-comp called "eyes" in which there is a "blink" and an open eye layer. Each is 1 frame long and sequenced within my pre-comp

2.) The "eyes" pre-comp resides inside of a composition called "head". I am using a time remap on the eyes pre-comp to extend it the full length of the head comp. I use toggle hold key frames to switch between the open eye layer and the blink layer

3.) When I preview the comp, it looks fine and I see my character close and open her eyes as expected.

4.) However, upon final render, only the blink registers and the character never opens her eyes. What gives?

This does not sound like a glitch to me, it sounds like a poor workflow. The usual procedure for doing something like this is to create a layer or a pre-comp with just the mouth positions, for example, each occupying a single frame then add an expression slider and tie that to the frame number. The keyframes for the slider are set to hold keyframes. Another technique is to tie the rotation of a null to time remapping, set the timeline to frames rather than seconds, and then use keyboard shortcuts to step through the rotation. This makes finding the mouth shape very easy. If you really want to stay working in time you can divide the rotation property by thisComp.frameDuration.

Here's a very old tutorial the shows how to set this up and gives some very good pointers.

If you have animated transitions between mouth shapes or blinks then you do not use hold keyframes for the slider but instead, duplicate keyframes between animations to hold the pose. Toggling hold keyframes on and off can be problematic.

Another way to approach the problem is to add an expression to time remapping that uses named markers. I saw one guy do this a long time ago. The expression was a little complicated with a bunch of If statements but it seemed to work fairly well for him and gave a visual representation in the timeline for the "oh" mouth shape. I thought it was a little more work than necessary.

If the preview is perfect and the render is missing frames or has odd movement you probably need to purge everything. Also, when I do this kind of animation I always render an image sequence instead of a movie. It's much easier to fine-tune and fix things in an image sequence. When you get that right rendering a compressed deliverable is very fast.

Without any screenshots of your comp and timeline as well as info on exact render settings, framerates etc. Nobody can tell you much.

Mylenium