Not sure if it helps but I have a workaround that works well for me. I manage all the bookmarks and sort them in the order I want via the EverSync website (https://www.everhelper.me/client/).

Then I sync these to Firefox on my laptop using the Firefox extension (works also for Chrome but I no longer use Google due to their selling my data/privacy issues).

On my iphone I DO NOT use the Eversync app, instead I let Firefox manage the sync from the desktop Firefox to the iOS Firefox....works a dream!!!

I have to say I am now a happy convert from Xmarks and have come to prefer it :-)

My mistake...I had auto sync turned on  in one browser that was overwriting bookmark sort in another browser. 

User error 🙄 not a bug