This reflects two things:
- In Brunei, there are only three vowels, so /i/ and /e/ are not distinguished.
- In Standard Malay, final /i/ in a closed syllable (such as in the root bersih) tends to be quite open, so it might be regarded as an allophone of /e/ rather than /i/.
Nevertheless, it is quite surprising that non-standard spelling is used on this sign, especially as kebersihan is quite common on signs around Brunei.