Quoting Wiktionary, the glyph origin of the character 「腺」 is as follows:
A 国字 (kokuji, “Japanese-coined character”) coined in the late 1700s–early 1800s by rangaku scholar Udagawa Genshin as a translation for Dutch klier (“gland”), as an ideogrammic compound (會意): ⺼ (“flesh; body”) + 泉 (“spring; fountain; source; producer of liquid”), together expressing the idea “part of the body that produces liquid secretions”.
And the origin of its pronounciation is described like so:
The reading sen is based on the kan'on of the 泉 base.
Which yields a 慣用音 of 「せん」(sen). However, it seems that other Sinitic and Sino-xenic languages' pronounciation of 「腺」 doesn't match with 「泉」, but rather 「線」. For comparison:
腺 / 線（私箭切） 泉（疾緣切） 標準漢語 xiàn quán 粵語（廣州話） sin3 cyun4 閩南語（廈門話） siàn（文）/ sòaⁿ（白） choân（文）/ chôaⁿ（白） 日語（漢音） せん（sen） せん（sen） 朝鮮語 선（seon） 천（cheon） 越南語 tuyến tuyền
Japanese seems to be the only language where the reading is consistent across all three characters. Why is this the case? Is there a connection between 「腺」 and 「線」? Or was there simply an error when 「腺」 was introduced to China/Korea/Vietnam?