With shift f3 open here is the warning when the lag spike occurs:

[13:29:31] [Render thread/WARN]: Something's taking too long! 'root.scheduledExecutables' took aprox 1258.6792 ms
[13:29:31] [Render thread/WARN]: Something's taking too long! 'root' took aprox 1272.0857 ms

This is reproducible every time by going over a chunk boundary, and occurs on both 1.16 vanilla, 1.16 with starlight, lazyDFU (on multiplayer), and sodium, and 21w11a.  I can attach a world if necessary.

I brought over the 1.16 chunk to 21w11a and have attached a debug log with four of the lag spikes.