This is a weird one - I've put it in the "Apex - Session State" category, but I'm not even sure that is correct...
I ran into this issue on my Search page, where sometimes the result would not be the part of the text containing the search term, but rather the start of the text.

Take a look at the two view queries below, which are identical except for a materialize hint in one of them.
In these views, I use a page item as the text to search for: "v('P1184_SEARCH_FOR') as search_for".
Turns out that the reason for the original view (which does not have the hint) to return the first part of the text, is the use of v('P1184_SEARCH_FOR') in the view query in combination with joining an Apex view - in this case apex_application_page_regions.
If I:
  change the v() function to a hardcoded string,
  OR remove the join to the Apex view,
  OR hardcode the search term in the regexp_instr functions,
  OR use a materialize hint,
it works fine.

So what's going on here? Well, I'm not sure really :-)
Maybe the Apex view switches to another session somehow, so that after the Apex view has been joined, the v('P1184_SEARCH_FOR') function then returns null (in regexp_instr)?
Something like that, maybe...
The simplest solution here was to add a materialize hint to the view. This ensures the value of "search_for" is only read once, and therefore stays the same "during" the query.

Item

Identification
	Sequence	10
	Name	P1184_SEARCH_FOR
	Display As	Display Only
User Interface
	Region	Parameters
Source
	Source Used	Always, replacing any existing value in session state
	Source Type	Static Assignment (value equals source attribute)
	Source value or expression	READ ONLY

Region

Identification
	Sequence	40
	Title	WITHOUT Hint
	Type	Report
Source
	Source Type	SQL_QUERY
	Table Name	TED_P1184_WITHOUT_HINT_V

View

View
	Name	ted_p1184_without_hint_v
	DDL	with iv_s as ( select v('P1184_SEARCH_FOR') as search_for , 200 as substr_n from dual ) , iv_a as ( select to_number(psc.used_by_25384_page_ids) as page_id , apex_escape.striphtml(psc.result) as region_source from ted_page_summary_caches psc join apex_application_page_regions apr on (apr.application_id = 25384 and apr.page_id = to_number(psc.used_by_25384_page_ids) and apr.region_name = 'Code') where psc.used_by_25384_page_ids = '1120' ) , iv_b as ( select iv_a.page_id , iv_a.region_source , greatest( regexp_instr(iv_a.region_source, iv_s.search_for, 1, 1, 0, 'i')-iv_s.substr_n/2, 1 ) as i , iv_s.substr_n , iv_s.search_for from iv_a cross join iv_s where iv_s.search_for is not null and regexp_instr(iv_a.region_source, iv_s.search_for, 1, 1, 0, 'i') > 0 ) select iv_b.page_id , iv_b.i , replace( regexp_replace( substr(iv_b.region_source, iv_b.i, iv_b.substr_n) , '^\s*', null, 1, 0, 'm') , chr(10), '<br/>') as region_source , iv_b.search_for from iv_b

Region

Identification
	Sequence	50
	Title	WITH Hint
	Type	Report
Source
	Source Type	SQL_QUERY
	Table Name	TED_P1184_WITH_HINT_V

View

View
	Name	ted_p1184_with_hint_v
	DDL	with iv_s as ( select /*+materialize */ v('P1184_SEARCH_FOR') as search_for , 200 as substr_n from dual ) , iv_a as ( select to_number(psc.used_by_25384_page_ids) as page_id , apex_escape.striphtml(psc.result) as region_source from ted_page_summary_caches psc join apex_application_page_regions apr on (apr.application_id = 25384 and apr.page_id = to_number(psc.used_by_25384_page_ids) and apr.region_name = 'Code') where psc.used_by_25384_page_ids = '1120' ) , iv_b as ( select iv_a.page_id , iv_a.region_source , greatest( regexp_instr(iv_a.region_source, iv_s.search_for, 1, 1, 0, 'i')-iv_s.substr_n/2, 1 ) as i , iv_s.substr_n , iv_s.search_for from iv_a cross join iv_s where iv_s.search_for is not null and regexp_instr(iv_a.region_source, iv_s.search_for, 1, 1, 0, 'i') > 0 ) select iv_b.page_id , iv_b.i , replace( regexp_replace( substr(iv_b.region_source, iv_b.i, iv_b.substr_n) , '^\s*', null, 1, 0, 'm') , chr(10), '<br/>') as region_source , iv_b.search_for from iv_b