Thanks Sam for your feedback.
I actually got a first bike delivered by Decathlon a few weeks ago. I thought the Lock-out was faulty, and got the bike exchanged (BTW, the Decathlon customer service was great and very easy to deal with to organise the swap).
A week later, I received a new bike. And the behaviour of the compression damper was exactly the same. I drove to my local Decathlon shop, and ‘pushed’ on the RockShox Reba RL they had in their inventory on similar bikes. Turns out, that none of the Reba RLs actually fully lock-up. They all felt ‘squishy’ when locked-out. And they all felt pretty inconsistent. That’s why I’m thinking of updating/hacking the compression damper myself.
I pulled out the compression damper out of my fork. Everything looks normal. In the unlocked position, the 2 holes in the compression damper line up (between the aluminium and the plastic part). In the spring-loaded locked position, the holes don’t line-up. The o-ring on the outside of the daper seems to seal well againt the fork leg.
My guess is that the leak between the aluminium and the plastic parts (the ones with the calibrated holes) is so large that the control of the oil flow is poor. I guess that more oil leaks between those 2 parts than through the 2 holes.
The rest of the fork works reasonnably well. And the fork is pleasantly rigid (especially compared to simalarly-priced Fox forks). So, I’m inclined to change the damper (either the compression damper, or the complete compression/rebound unit) for a higher spec model. But I don’t know which model will fit? Charger? Charger 2? Are they available for 29″ 100mm Rebas? and especially, which damper models actually offer a ‘real’ lock-out?