In the current and previous snapshots It is possible to create a strider which can fly. This is due to the existence of 2 different types of striders who follow very different rule sets. 

The first type of Striders that I will discuss are those that can be spawned in via creative mode. These behaviors are exhibited by any Striders which are spawned via creative mode as well as Striders who naturally spawn carrying a grown zombified piglin(ii) 

-These Striders normally will climb a a 1 block high hill assuming they are at exactly 1 block lower. Lava to 1 block high above lava will not work (Some speed boosting exceptions exist but are not relevant to this discussion). 

-These Striders will normally experience a buoyant force pushing it to the top of any lava they are in. This means if one of these Striders walks into a lava column they will float up to the top of it. 

-If these Striders are led off of a cliff regardless of height they will fall until they hit either a hard surface and experience fall damage or until they hit lava in which case they will sink rapidly then be pushed back up to the surface due to "buoyancy". 

The second type of Striders are those that spawn naturally. This behaviour has been noted in all Striders that naturally spawn without a saddle(iii), a Strider which naturally spawned with a saddle(iv) as well as a Strider that naturally spawned with a saddle that was standing on top of another Strider(v). 

-These Striders are incapable of climbing a 1 block high hill from lava or from exactly 1 block below it. If they hit such a hill they will merely walk towards the hill without ascending. 

-If these Striders enter a lava column they will merely stay stationary in it not experiencing any "buoyant" force. However they do experience said buoyant force from lava source blocks.

-If these Striders are led off of a cliff instead of falling they will merely continue walking in the air. They treat the air seemingly as a solid block which is notably not lava (the Striders will start shivering in mid air). 

The "Flying": 

-Notably as the naturally spawned striders are still able to experience buoyant force from lava source blocks by placing an effective staircase of lava source blocks you are able to get the strider to any specified height and then walk anywhere desired in the air. 

-FYI as far as I have found ( I haven't tested Water flow or pistons) there does not appear to be a way to lower these Strider's altitude.

Hopefully this information will help you improve a/o homogenize the Striders behavior as they are one of my favorite new additions from the new Nether update. Good luck and thanks for making this game. 

A Y-L

Testing Notes

Using eggs. This has been tested 10-12 times

(ii) This is from a sample size of 1 so hard to guarantee (I'm sorry they are really rare).

(iii) With a sample size of around 10

(iv) With a sample size of 1

(v) With a sample size of 1. Notably in this case upon the Strider beneath being killed the Strider on top did drop to lava level.