It's not a bug, when you Set the marker, it aligns with a transient close to the play head but not always on it. It's positioning will affect all of the automatically created markers either side of it until it reaches the next manually created marker.

Manual adjusting beat grids is a fine art :-)

You'll have to excuse my ignorance - what is a transient?
And why does it not align to the playhead? Surely there's no point in manually adjusting beatgrids if you can't place the warp markers where you want them? Is there a way to change this, because I have a lot of tunes that the automatic beatgridding has not worked on.

In simple terms, a musical peak... Serato is trying to help you by placing the marker where it thinks is best. I agree though, I'd rather it place it where I want...

If it doesn't, once Set, nudge it along to the right position using the grid adjust buttons.

Thanks for explaining. I'm quite disappointed - first that I'm having to manually adjust so many beatgrids in the first place, and second, that Serato makes it so hard to do! Is this something that would be likely to change if I were to put in a request, do you think?

You can always request it but likely not. The current method has been that way for 5 or so years and is better than competitors like Traktor. Most people that run into an issue with beat griding, simply grid the beginning or ending, to allow mixing in and out. however if there's a bunch of variance, it's probably best just to pop the file into a DAW (like Ableton) and quantitize it if you are planning on doing long mixes.

One suggestion i have is try to use clear transients. i usually try to do mine each measure or so, but sometimes the bar before or after is a better candidate. Also think about how you plan to use the song, if you aren't planning on mixing throughout the entire song, just concentrate on properly griding the sections you know you'll use.

It would be interesting to understand the thinking behind not placing the marker where the play head is @Serato, any comment?

TBH, once you get you workflow sorted, it's a pretty decent system to use. I have to manually grid a lot of tracks but on average a 5 minute long track will take me around 10-15 minutes max.

What would really help is a metronome so you could hear if your makers were on the beat. The metronome should click, every time the the grid marker passed under the play head. This would help where the waveforms are tightly packed or during an acapella section and it's not easy to pick out the "beat".

Another tip to avoid too many adjustments...

You can just leave some songs' downbeat on either second, third, or fourth count..
You can mark the first bar that switched to those odd downbeats if you have spare cue points.

The most common measure is 4/4 but some songs were made to have 2/4 or 3/4 bar inserts intentionally.

Original version of Despacito is a good example.

I absolutely hate it when my the software put a marker in different place. It should always place it where the play head is. I personally feel that this is a bug in Serato that they have just never bothered to address!