Inconsistent data types (Text vs Variant) in List of Map output

The {} come up as List of variant. To avoid having list of variants in the output cast these {} to string using tostring({}). That will give expected output. 

Try this: 

a!localVariables(
local!indexval: index(ri!userFilters, "userFilter", null),
local!filters: split(local!indexval, ";"),

/* Find the segment containing AccountName */
local!accountSegment: a!forEach(
items: local!filters,
expression: if(find("name=AccountName", fv!item) > 0, fv!item, null)
),
local!accountRaw: index(
filter(not(isnull(fv!item)), local!accountSegment),
1, null
),

/* Find the segment containing Product Category */
local!productSegment: a!forEach(
items: local!filters,
expression: if(find("name=Product Category", fv!item) > 0, fv!item, null)
),
local!productRaw: index(
filter(not(isnull(fv!item)), local!productSegment),
1, null
),

/* Parse AccountName values */
local!accountValues: if(
isnull(local!accountRaw),
{},
a!localVariables(
local!start: find("value=[", local!accountRaw),
local!block: mid(
local!accountRaw,
local!start + 7,
len(local!accountRaw)
),
local!clean: substitute(
substitute(local!block, "]", ""),
"isArray=true,", ""
),
filter(
not(isnull(fv!item)) && fv!item <> "",
a!forEach(
items: split(local!clean, ","),
expression: trim(substitute(fv!item, """", ""))
)
)
)
),

/* Parse ProductCategory values */
local!productValues: if(
isnull(local!productRaw),
{},
a!localVariables(
local!start: find("value=[", local!productRaw),
local!block: mid(
local!productRaw,
local!start + 7,
len(local!productRaw)
),
local!clean: substitute(
substitute(local!block, "]", ""),
"isArray=true,", ""
),
filter(
not(isnull(fv!item)) && fv!item <> "",
a!forEach(
items: split(local!clean, ","),
expression: trim(substitute(fv!item, """", ""))
)
)
)
),

/* Single combined map */
a!map(
AccountName: local!accountValues,
ProductCategory: local!productValues
)
)

Issue
Two separate maps
isArray=true
Empty strings in output

Your if() returns empty List of Variant when the condition fails, and a!forEach makes one map per filter (not merged).
Type the empty fallback and merge after the loop.

See below - Returns one map with both keys as List of Text.

a!localVariables(
  local!filters: split(index(ri!userFilters, "userFilter", null), ";"),
  local!parsed: a!forEach(
    items: local!filters,
    expression: a!localVariables(
      local!vStart: find("value=[", fv!item),
      local!vBlock: if(isnull(local!vStart), "", mid(fv!item, local!vStart + 7, len(fv!item))),
      local!values: a!forEach(
        items: split(substitute(local!vBlock, "]", ""), ","),
        expression: trim(substitute(fv!item, """", ""))
      ),
      a!map(
        type: if(find("name=AccountName", fv!item) > 0, "A", "P"),
        values: local!values
      )
    )
  ),
  a!map(
    AccountName: index(local!parsed, wherecontains("A", local!parsed.type), a!map()).values,
    ProductCategory: index(local!parsed, wherecontains("P", local!parsed.type), a!map()).values
  )
)