In my phonetics class yesterday, I asked my students to transcribe 'complicated'; and half of them used /æ/ instead of /eɪ/ in the third syllable. The fact that they cannot differentiate between /æ/ and /eɪ/ confirms that speakers of English in Brunei tend to merge the TRAP and FACE vowels.
Why should this be? It is quite different form Singapore, where people tend to have a close monophthong for the FACE vowel, and this vowel is quite distinct from the TRAP vowel. (The latter tends to be merged with DRESS, and both are pronounced as [ɛ].)
The first thing to notice is that there is a systematic link between /æ/ and /eɪ/ in English. For example, 'vane' has /eɪ/ while 'vanity' has /æ/, and the same is true for 'sane'/'sanity', 'profane'/'profanity', 'nation'/'national' and many other pairs of words. And it is not surprising if some speakers use the same vowel in both the base form of the word ('vane', 'sane', etc) and its derivative ('vanity', 'sanity', etc.). Furthermore, there are a few words in which there is variability: in both 'patent' and 'patriot', some speakers have /æ/ while others have /eɪ/.
However, this does not explain why there is a different pattern in Singapore and Brunei. And for this, we must consider the dominant indigenous languages.
Most Singaporeans are Chinese, so we would expect their English to be influenced by Chinese. Now, Chinese has lots of words with the /ei/ diphthong, such as 给 (gěi, 'give') and 黑 (hēi, 'black). However, there is no such diphthong in Malay. In Malay, /ai/ can occur at the end of words such as cukai 'tax' and sungai 'river' (though we can actually argue that these are monophthongs followed by an approximant; see here), but the /ei/ diphthong does not occur.
Then we might note what happens to English words with /eɪ/ when they are borrowed into Malay. In most cases, /a/ is used: radio, status, stadium, agensi and templat all have /a/ while the original English has /eɪ/ (though there are some exceptions: kek 'cake' and ejen 'agent' bother have /e/ rather than /a/).
Next, we might note that, according to my dictionary, 'plate' becomes plat in Malay (as in plat nombor), while 'plaque' becomes plak, so we can see that though the vowel is different in these two words in English, it is the same in Malay.
It seems that it is probably this influence from Malay, particularly the way that English words are borrowed into Malay, that influences the pronunciation of English in Brunei.