I do not play tumble based character right now (favored soul that does not need it), so I just tested it with 1 point invested (plus some bonus from item and dex, it was positive in total, so I was able to roll, not hop) and no mobility feat.
I had 2 charges and when tried to make quick double tumble forward for longer distance (some situations may require it like long traps, large enemy, groups of enemies, multiple projectiles and aoe), then I ended up with no second tumble (2 charges were still used) or teleported back after second animation (I was somewhere between position of 1 and 1.5 tumble) or had too long lag between first and second (it was not smooth enough, so you can get hit during that delay by trap or enemy, but possibly still defended by dodge or reflex save).
Maybe 2 vs 3+ charges or points in tumble or forward vs backward makes difference (I mean is bugged).
You may compare with other character if you can.
I do not promote using tumble for increased movement speed.
Dodge and reflex save bonus is nice idea, but using tumble should not break combat flow, just like you are not limited with jumps, so when you need to, then you do not end up staying in place and do not have to check charges each time with block first (there is no time for this during fast paced combat on higher difficulties when solo against all, because you auto react instantly with expected evasive movement or get hit).