That underlined note was placed more to highlight the changes to TAG than ECM. As under previous TAG implementations, it provided lock-on time boosts. But this boost was simply rolled over into the core weapon lock functionality whenever a target is in direct LOS. Essentially, that particular note is there to stress that there is no longer any way to augment direct fire LOS locks. It's all rolled into the base package.
ECM applies a multiplier to indirect lock-on times, with the base lock on time being based on the target's relative distance within the firer's max sensor range. So at closer ranges, ECM would only slightly hamper lock-on times, but at extreme ranges, it will make indirect lock-ons particularly challenging. It should not affect locks within direct LOS.
Without any video, tough to tell if the behavior your encountering is intentional or not, as there could be any number of things in-play:
- While you might have been in the open, you could have had friendly or enemy 'Mechs intervening in your direct LOS to the target. Friendly and Enemy 'Mechs are considered LOS blockers. While this primarily comes into play in regards to direct fire vs. indirect fire arcs behind team mates so you won't friendly fire someone who is screening for you, It does also mean that any lock-on attempted against a target with an intervening friendly or enemy 'Mech, it would be considered out of LOS, and there for, would result in a longer lock-on time if the target was benefiting from an ECM effect.
- Another one I see often is attempting locks against targets with high amounts of Radar Dep skills. If a target has 100% radar dep maxed out, when the radar dep kicks in and drops your sensor lock against a target, it also completely resets any lock-on weapon attempts against a target. So even if both the firer and the target are in direct LOS, if the target has 100% radar dep and jumps between intervening terrain, it would also re-set your lock-on giving the impression of longer lock-on times when in reality, its due to your locks being reset whenever his radar dep kicks in.
- If you are put into a low-signal state (under the influence of enemy ECM, Enemy Jamming tower in Incursion activated,) Your Lock-on weapons can no longer lock-on to targets. You will have to dumb fire them until you are out of your Low-signal state.