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Tried /give and it did not trim the space in a command block as you said, thank you for clarifying.

/say's argument is a greedy string, it accepts any and all text. Try with a non-greedy command (e.g. /give)

The reason it works in chat is because spaces get trimmed (removed at the end, and multiple after eachouter turned into 1) from chat. Meaning this is actually MC-65034.

Cannot reproduce this with a simple /say comamnd, what command did you use exactly?
2024-01-19_13-57-21.mp4