I don't know if they somehow fixed the issue already, but I haven't had this issue anymore in a while so far.
I do remember having the file (libgstflac.so) in the wine runner as well, but a different size to the system gstreamer's libgstflac.so. So I just replaced the one in the wine runners (both from the lib and lib64 folders). Can't tell if it worked or they just fixed it lol.
I have been having a similar problem and I had a "MAYBE" working solution so far.
OS: Arch Linux
CPU: i5-11600K
GPU: Radeon 6700XT
Played like 3 games so far and haven't got that lag/sped up game issue yet.
The main issue was the libgstflac.so under the gstreamer-1.0 folder of the wine-ge-lol-8.7-1-x86_64 runner inside both "lib" and "lib64" folders. This was the log file that lutris gave me back then:
...
(wine:26885): GStreamer-WARNING **: 22:17:24.871: Failed to load plugin '/home/(username)/.local/share/lutris/runners/wine/lutris-ge-lol-8.7-1-x86_64/lib64/gstreamer-1.0/libgstflac.so': libFLAC.so.8: cannot open shared object file: No such file or directory
(wine:26885): GStreamer-WARNING **: 22:17:24.872: Failed to load plugin '/home/(username)/.local/share/lutris/runners/wine/lutris-ge-lol-8.7-1-x86_64/lib/gstreamer-1.0/libgstflac.so': /home/(username)/.local/share/lutris/runners/wine/lutris-ge-lol-8.7-1-x86_64/lib/gstreamer-1.0/libgstflac.so: wrong ELF class: ELFCLASS32
...
The issue then seems to be that there's no libgstflac.so or a wrong version of it inside the lib folder of lutris-ge-lol (for some reason). So, these are what I did.
/home/(username)/.local/share/lutris/runners/wine/lutris-ge-lol-8.7-1-x86_64/lib/gstreamer-1.0/libgstflac.so
/home/(username)/.local/share/lutris/runners/wine/lutris-ge-lol-8.7-1-x86_64/lib64/gstreamer-1.0/libgstflac.so
So far, no more errors regarding the plugin anymore. Haven't got that lag issue yet, but need more sample size (maybe you guys can try that and increase the sample size altogether).
Please let me know how it goes. Mine's been running ok so far after the workaround with libgstflac.so. (if the file exists in the wine runner already, rename it for backup and replace it with the system's version of libgstflac.so found in /usr/lib/gstreamer-1.0/ ) I straight up copied in/replaced both of the ones under the lib and lib64 folders for the wine runner with the same system libgstflac.so file because I got sick of it.
I don't know if it worked because of the workaround or if someone fixed it for arch linux though. LOL. Good luck!