+0-180 degrees of mesh layer are displaying as horizontal lines [QGIS]

I have been trying to display weather data from ECMWF in QGIS on top of a base map, but half of the heatmap rendered from the mesh layer (imported via grib) displays as straight horizontal lines.

I am using QGIS 3.12.1
The base layer are XYZ tiles (Pseudo-Mercator EPSG:3857).
The overlay was imported from ECMWF in .grib format and imported as a mesh layer.

The two layers are clearly aligned, as the windspeeds clearly drop around the shape of South America and match the base layer precisely.
When I also display vector arrows for wind speed, they seem to cover the whole map:

Why is the heatmap imagery not displaying correctly at +0-180degrees, and how can I fix this?