My guess is Lionel Shrike faked his death for some larger as yet unknown purpose.
Short answer is that he used a trick safe developed by the Eye and has been hiding in the backgrounds for the past two movies. This is corroborated by the second movie where Max Shrike finds a prototype safe at the Macau magic store and is able to withstand the same scenario his father was put through when Tressler and Mabry try to kill him. Meaning the safe wasn't made by Elkhorn, it had a trick opening from the inside, and is easy to escape without detection using SCUBA gear. As to why, the answer seems to be about initiation into The Eye and whether its' political beliefs would call for such a measure. Due to their anti-authoritarianism beliefs, they would need to carefully vet every candidate and make sure they would be up to the effort. Hence the reference the Interpol agent makes to new recruits needing to carry out blind commands. It could be Lionel Shrike had to fake his death to show his devotion, much like Jack Wilder.
Here's a full theory on it: https://www.reddit.com/r/FanTheories/comments/a0ve4b/now_you_see_me_too_lionel_shrike_faked_his_death/