albeit GIS does suggest that cycling on water is possible
What does that mean? Apparently bikes are not allowed to go over the water (in OSM!). You can find out on https://openstreetmap.org
Do you know how isochrones are calculated? If not, I recommend reading up a little. Basically, it gives you a point cloud from routing calculations and to get a polygon you have to create a Concave Hull. The latter is dependent on a few parameters, one of them being a minimum distance below which it doesn’t look for nearer nodes (very abstracted explanation). So, for different ranges of isochrones in our current implementation this parameter changes. That leads rarely to more or less inconsistent behaviour. That’s all there is. The routing is of course fine, it’s just the polygon/concave hull generation. Not a thing to worry about, it’s rare and obviously not very impactful.