Thanks for the data sheets Leo. Unfortunately they don't shed much light on this situation though... the sealer and top coat seem nearly identical according to the description, the viscosity, and the solids content. The VOCs is a little different.
The sealer has a high solids content, even though the viscosity is low, which contributes to the ridged build-up around the pores. Thinning it enough to avoid the build-up reduces the solids content down to the wash-coat thickness - not enough to give you a good base for the top coats. That means you'l have to spray an extra coat or two of thinned sealer which drives up the cost of the finish a lot.
The properties of a good sealer are excellent flow, leveling, adhesion, and sand-ability. The TDS for the sealer doesn't say anything about easy sanding which is unusual, and from your description it sounds like it doesn't flow and level well.
Personally, I'd switch to another product instead of trying to make this one work in this situation. ML Campbell may tweak the formulation over time and get it to work better with ring porous woods or they may simply recommend a different product. For now, the Eurobuild high solids sealer is probably a better choice with oak. If you're open to trying new products, the LBA-38 sealer from Milesi is exceptional.
You should be able to spray one coat of sealer, sand smooth easily, and apply two top coats and get great results consistently. Any extra fussing is not worth the effort/time/money.